In one dimensional digital communication systems the transmitted waveform is formed by adding time-shifted versions of a basic pulse shape. The amplitude of this pulse is adjusted according to the data being sent (e.g. binary phase shift keyed). In multi-dimensional digital communication systems (e.g. Quadrature Amplitude Modulated) multiple pulse streams are generated according to the data. To minimize the bandwidth of the transmitted waveform and thereby secure that the transmitted waveform does not interfere with other systems operating in a nearby (frequency) channel, the pulse shape used must have a time duration which spans several symbol intervals. That is the pulse associated with one data symbol will overlap pulses associated with adjacent data symbols. Certain data sequences will cause these overlapping pulses to add constructively producing large peaks in the transmitted waveform, while other data sequences will cause these overlapping pulses to cancel one another producing small values of the transmitted waveform. Amplifiers that are used to boost the power of the transmitted signal just prior to transmission work best when the signal remains at a fairly constant level. Large peaks in the transmitted signal lead to inefficient usage of the power amplifier which in turns wastes precious battery life.
Battery operated communication devices employ a variety of techniques to save battery energy in order to prolong the operating life of the battery. Increasing the efficiency of power amplifiers is one technique that designers utilize to prolong the operating life of a communication device. Another scheme by which battery energy may be saved is the use of another power-efficient modulation technique. Various modulation techniques have different associated peak-to-average power ratios. In general, it is highly desirable to have a peak-to-average ratio as close to zero dB as possible. However, many existing modulation formats result in relatively high peak-to-average power ratios. Two commonly used modulation formats are Phase Shift Keying (PSK) and Quadrature Amplitude Modulation (QAM). The former uses a signal constellation where all data symbols have the same magnitude while the latter varies both the phase and magnitude of the individual data symbols. Binary signaling is a special case of PSK (i.e. BPSK). In both modulation formats, the peak-to-average ratio depends upon the pulse shape used.
Quadrature Amplitude Modulation (QAM) utilizes both the phase and amplitude of a carrier to transmit information and hence has the potential to generate a higher peak-to-average power ratio. Indeed, experiments have demonstrated that, for example, a sixteen symbol PSK constellation enjoys a 3-4 dB improvement in peak-to-average power ratio over a 16 QAM signal. However, this gain in efficiency improvement is accompanied with a 4 dB loss in sensitivity. Due to this loss of sensitivity, many system designers prefer to use the QAM modulation format despite its degraded peak-to-average power ratio.
Referring to FIG. 1, a communication device is shown as is presently available. FIG. 2 shows a phase and magnitude trajectory of a complex baseband 8 PSK signal. In other words, this figure represents the transition from one symbol to the next as the generated data changes state. A filter that is used to limit the sideband noise produces undesirable overshoot as shown by reference 202. This overshoot 202 contributes to an increase in peak power which results in an increase in the peak-to-average power ratio. This increase in the peak-to-average power ratio forces a designer to design an amplifier that can tolerate the maximum peak power which in turn renders the power amplifier more expensive to produce. In addition, the increase in peak-to-average ratio reduces the power efficiency of the power amplifier.
In the design of portable communication devices, the aim of a designer is to utilize efficient components at the lowest possible price. Power amplifiers have traditionally been some of the most expensive components of a communication device and have often resisted attempts aimed at lowering their cost. One parameter that is directly related to the cost of amplifiers is the peak-to-average power ratio. This is because the designer is forced to employ an amplifier that can handle peak powers significantly larger than the average power. It has therefore been the goal of designers to reduce peak-to-average power ratios as much as possible without degrading other performance parameters. There is therefore a need for a modulation scheme that would have minimum peak-to-average power ratio without suffering other performance degradation.